1. Field of the Invention
The present invention relates to a substrate used for manufacturing a semiconductor device and a method for manufacturing the semiconductor device from the substrate. More specifically, it relates to a semiconductor-on-insulator substrate used for fabricating thereon a semiconductor device in which both a semiconductor element having amorphous silicon or polycrystalline silicon active film and a high electron mobility semiconductor element having a higher performance are formed together on a common insulating substrate, and to a method for manufacturing such a semiconductor device from the substrate.
2. Description of Related Art
Such a semiconductor device for use in a high performance liquid crystal display (LCD) is known which has thin film transistors composed of a hydrogenated amorphous silicon active layer formed on a glass substrate in a pixel area. This type of semiconductor device has an advantage that leakage current flowing between the source and drain regions of the thin film transistor is small as a result of employing the hydrogenated amorphous silicon film formed on the glass substrate.
With the semiconductor device as described above, when a drive circuit for driving an array of thin film transistors in the pixel area is also formed of thin film transistors having an amorphous silicon active layer, the drive circuit does not have sufficiently high electron mobility for driving the arrays of the thin film transistors in a high performance. Accordingly, in a conventional high performance LCD, a semiconductor device for a drive circuit formed of transistors having a single crystalline silicon active layer is additionally provided to the semiconductor device having pixel elements. The two semiconductor devices are connected by wires between terminals of both the devices, resulting in a low reliability and a high cost.
Another type of semiconductor device for use in a LCD has an integrated circuit comprising a pixel area and a drive circuit area both having thin film transistors of polycrystalline silicon active layer, which have a higher current drivability than the thin film transistors having a hydrogenated amorphous silicon active layer have. When thin film transistors having polycrystalline silicon active layers are employed in a drive circuit area, however, since the thin film transistors having polycrystalline silicon active layers do not have sufficient high electron mobility, this type of semiconductor devices has drawbacks in that LCD's having a large size cannot be obtained.
Still another type of semiconductor devices for use in a LCD has been proposed. In this type, an array of pixel elements comprising thin film transistors having a hydrogenated amorphous or polycrystalline silicon active layer are disposed on the central area of an insulating substrate and the drive circuit for driving the array of pixel elements and comprising thin film transistors each having a high electron mobility is disposed on the peripheral area of the insulating substrate. This type of semiconductor device is obtained by annealing a semiconductor film composed of hydrogenated amorphous silicon or polycrystalline silicon by use of laser beam scanning on the peripheral area of the substrate.
With the last type of the semiconductor devices as described above, however, annealing by scanning a laser beam costs a large amount of time, so that production cost of the semiconductor device is extremely high due to the low through-put of the annealing, and the uniformity and reproducibility of the semiconductor device are not satisfactory. Moreover, when a semiconductor film of hydrogenated amorphous silicon is exposed to laser-annealing, hydrogen gas contained therein drastically expands, thereby destroying the recrystallized film.